The present invention is related to centrifuge rotors and, more particularly, is directed to a centrifuge bowl rotor with a transparent lid capable of being sealed to the rotor in such a manner that any internal liquid sample which is accidentally aerosolized within the rotor will be contained within the rotor.
In order to more fully understand the circumstances involved when a liquid becomes aerosolized attention is directed to the following discussion relating to the causes and characteristics of an aerosol especially with respect to a centrifuge rotor. An aerosol is any more or less stable dispersion of fine particles in a gas. The particle size distribution and the exact meaning of stable are not universally defined but are usually based on consideration of the immediate aerosol related application or investigation. For present purposes, particles in the size range of 15 .mu.M and lower have sufficiently low settling rates to be of concern. Fine aqueous droplets in a low humidity environment will tend to form smaller or even fully dried solid particles which will have even lower settling rates. The particle size range of most immediate hazard to humans are particles of 2 .mu.M to 8 .mu.M, which penetrate and are retrained in the aveoli of the lungs on inhalation. It is also this size range and somewhat larger which can be expected to become rather thoroughly distributed throughout a laboratory within minutes or a few hours of an aerosol release. Both inhalation and surface contamination are of concern.
Any sort of liquid splashing or bubble breaking gives rise to aerosol formation. Due to the high centrifugal field in a centrifuge rotor the liquid released from a suddenly failed centrifuge tube literally crashes against the rotor wall. The closest familiar analogy might be the splashing of water at the base of a very high waterfall, where the mist formation is a matter of common observation. Liquid particles will to a considerable extent be sedimented in the centrifuge rotor so that given a sufficiently long run, all of the aerosol will be collected providing there was no way for it to escape from the rotor. However, it must be noted that in many applications runs are quite short, and broken tubes also lead to run termination by excessive vibration. It follows that it is desirable for the operator to have easy means for observing tube breakage so that the rotor will not be opened while there may still be aerosol in it, except inside a suitable biocontainment hood.
A classical means for generating aerosols is by feeding a liquid to a spinning disc or spray head. Clearly, a centrifuge leaking a liquid from a large diameter rotor running at high speed is a very good aerosol generator. The liquid is pulled from the rotor by its centrifugal weight in tread-like streams. A short distance from the rotor the streams become unstable, depending on surface tension, density and viscosity and break up into rather uniform size droplets. These droplets, which in the instance of most practical centrifuges, are quite large and always accompanied by a small trailing cloud of very small particles. It is these latter particles which are of great concern.
One of the more important concerns in clinical and research laboratories is the escape of any hazardous materials during experimentation. Quite often, it is necesary to place a container of hazardous biological material having dangerous viruses in a centrifuge rotor for a centrifugation run to separate particular strains of viruses. However, for the safety of the individuals in the laboratory, it is necessary to provide all possible safety precautions to prevent any accidental contact between the personnel in the laboratory and the dangerous virus or other biologically active material.
The containment of a liquid sample is normally assured when the sample is placed within a sealed centrifuge tube and placed within a rotor having a cover lid. But, in some instances, the centrifuge tube or sample holding container may have a defect which will result in rupture during high speed centrifugation, causing the leakage of the dangerous material into the rotor itself. Depending upon the design of the rotor, some of the liquid may be aerosolized and escape from the rotor if it is not properly sealed or otherwise contained.
In some cases where a lid is provided to close the rotor, the lid is typically made of a solid metal or other opaque material which does not permit the visual inspection of the contents of the rotor after centirfugation. Consequently, it is almost impossible for the operator to tell whether or not any of the centrifuge tubes or containers have been broken during the centrifugation run which would result in exposure of the operator to a possible hazardous biological material when the lid is removed from the rotor. It is extremely important that any sealing means between the lid and the rotor provides essentially complete sealing under both static and running conditions, so that there is complete containment of any hazardous material within the rotor.
One of the more important requirements of the lid of the rotor is that it be able to withstand any hydraulic forces that may be applied to it due to the leakage of a fluid sample within the rotor.
The hydraulic head that will be generated by a liquid in a centrifuge depends on the radial depth of the liquid and the average centrifugal field. If the liquid is rotating as a "solid" plug (forced vortex), then with respect to the rotor it generates a hydrostatic pressure; however, because it is moving with respect to ground, it is in reality a hydrodynamic pressure. Furthermore, especially under leaking sample tube conditions, the free liquid in the rotor may not rotate as a solid plug and instead may be, on average,tending to rotate faster or slower than the rotor. In the first instance, the hydraulic force will be higher and in the latter instance lower than the "solid" plug condition. Which condition may exist depends on the way the tubes are leaking. The hydraulic forces of a free liquid sample in a rotor can become quite substantial when operating at high speeds and, therefore, may be substantial enough to break the seal which may have been established between the rotor lid and the rotor. As a consequence, the escaping liquid sample from within the rotor may be thrown outward within the centrifuge, generating an aerosol in the process, and causing contamination of the centrifuge area.